Ethylene formation is the only possible channel for mass loss of 28 in \([py+7H]^+\)  with/without geometry constraints during heating as extracted from the MD trajectories in Fig. REF  (a).
The added proton easily jumps from one carbon atom to another in the molecule and ethylene is only ejected when the proton is bonded to the carbon next to the desorbed C\(_2\) H\(_4\)  group.
The same hydrogen migration is thought to contribute to H\(_2\)  loss [1]} and CH\(_n^+\) (n=4-6) fragments[2]} from PAHs.
Further confirmation of ethylene formation is achieved through the IR calculations of the fragment (m/z=181) which are shown in Fig. REF  (b)-(f).
To explore the configuration space of the fragment (m/z=181), a global optimization method, evolutionary algorithm (EA) [3]}, [4]} is used to generate as many candidates as possible.
Selected candidates from the EA approach and one candidate from the MD runs corresponding to the ethylene formation are subsequently screened by harmonic IR calculations.
The infrared features of the candidate from the MD search (blue color, Fig. REF  (b)) have better agreement with the experiment (Fig. REF  (f))
than the most stable configuration (global minimum, plotted in green color, Fig. REF  (e)) and other candidates (gray color, Fig. REF  (c) and Fig. REF  (d)) found by the EA search.
The band assignments of the MD candidate were achieved by visualizing its normal modes, see Fig. REF .
The comparison between the IR spectrum of the parent ion \([py+7H]^+\)  and that of the fragment (m/z=181) is shown in Fig. REF .
A computed band of the MD candidate at 1140 cm\(^{-1}\)  is due to the in-plane bending of aromatic C-H.
A series of bands in the 1200 - 1500 cm \(^{-1}\)  region have contributions from C-C stretching, C-H in-plane bending and -CH\(_2\) - bending.
The computed band positions at 1229, 1301, 1375, 1417 and 1488 cm\(^{-1}\)  in Fig. REF  (b) agree well with the IRMPD peaks at 1230, 1300, 1380, 1415 and 1485 cm\(^{-1}\)  in Fig. REF  (f).
The bands above 1500 cm\(^{-1}\)  involve aromatic C-C stretching and aromatic C-H in-plane bending.
The computed peak at 1573 cm\(^{-1}\)  is the closet one to the 1580 cm\(^{-1}\)  IRMPD band, even though their relative band intensities differ a lot.
For other three isomers, their bands may fit well with some of the IRMPD bands.
However, they either have bands that are absent in the IRMPD spectrum or fail to reproduce some IRMPD peaks.
Given that the agreement between Fig. REF  (b) and Fig. REF  (f) is already good,
we refrain from constructing a blended IR spectrum composed of contributions from numerous isomers.
Such an attempt was done by [5]} in another H-functionalized pyrene system and was found challenging as it requires choices to be made for the number of isomers to include and for their relative weights.
Based on the good match between Fig. REF  (b) and Fig. REF  (f), we use the MD-determined isomer (Fig. REF  (b)) in the reminder of the paper.
It has 0.47 eV higher energy than the global minimum.
This implies that the the fragment (m/z=181) might stay in a local minimum region without opportunities or enough energy to jump into the global minimum after losing ethylene from \([py+7H]^+\) .
Combining the results from MD and IR calculations, it is clear that ethylene is responsible for the mass loss of 28 for \([py+7H]^+\) .
The same condition for ethylene formation is valid for \([py+5H]^+\) , but the difference is that the yield of ethylene is much lower than for \([py+7H]^+\)  as observed from the extremely low intensity of the fragment (m/z=179) in Fig. REF  (a) and (b).
<FIGURE>